a) Field of the Invention
The present invention relates to a shaft-fixed-type motor used in rotary polygon mirror type optical scanning devices and the like. More specifically, it relates to an improvement of a structure of fixing a fixed shaft to a motor frame.
b) Description of the Related Art
A shaft-fixed-type motor used in rotary polygon mirror type optical scanning devices and the like is disclosed in, for example, JP No. H6-17687. In the shaft-fixed-type motor, the fixed shaft is fixed to a motor frame such that a shaft edge thereof is shrinkage-fitted or press-fitted to a hole formed in the motor frame. A rotary member is rotatably supported around an outer circle of the fixed shaft. Around the outer surface of the rotary member, a rotor assembly is integrally installed. Further, a stator assembly is arranged facing the rotor assembly. Also between the fixed shaft and rotary member, a hydrodynamic bearing mechanism is provided by cutting hydrodynamic pressure generating grooves on the outer surface of the fixed shaft. Upon driving the motor, the rotary polygon mirror attached to the rotary member turns together with the motor, and then an incident laser beam is reflected at the rotary polygon mirror and polarized in a predetermined direction.
In addition, U.S. Pat. No. 5,555,211 has disclosed a magnetic disk driving motor in which a fixed shaft is fixed to a motor frame using screws.
When fixing the fixed shaft to the motor frame in the above mentioned manner, however, there are the following problems to be solved.
It is desirable to fix the fixed shaft with a certain angle with respect to the motor frame, that is, with a right angle. However, when using a shrinkage-fitting or press-fitting fixing method, it is highly possible in practice that the shaft be slightly inclined with respect to the perpendicular direction. For example, a stress generated when the fixed shaft is pushed in the motor frame is applied onto the fixed shaft and motor frame, and consequently a deformation occurs on the whole or the fixed shaft inclines. The inclined fixed shaft will be a cause of surface vibrations on the rotary polygon mirror attached to the rotary member during the rotation. As a result, scanning cannot be performed in a direction targeting the laser beam reflected at the vibrated mirror surface.
In addition, these fixing methods require more strict control of the dimensions of the shaft end of the fixed shaft and the hole in the motor frame. If the hole diameter is too large, for example, the fastening strength of the fixed shaft is decreased. Consequently the fixed shaft may come off the motor frame during the driving.
Furthermore, if the shrinkage-fitting or press-fitting of the fixed shaft to the motor frame is carried out in an improper manner and therefore the product needs to be removed as defective, it is not economical because the expensive fixed shaft needs to be abandoned together with the motor frame.
Also, in a method of fixing the fixed shaft on the motor frame with a screw, a screw hole needs to be formed in the fixed shaft, requiring machining, and then producing the machining waste when forming the screw hole. If the machining waste is not removed completely, the remnant may invade the bearing, causing noise and poor rotations during the motor operation.